1. Field of the Invention
The present invention relates to a process for the preparation of isocyanate compounds which comprises a two-stage reaction using a diamine and dimethyl carbonate as the starting materials.
2. Description of the Related Art
An isocyanate compound is an industrially valuable compound, and especially, a bifunctional isocyanate is valuable as the starting material of a polyurethane.
Currently, an isocyanate compound is industrially prepared by reaction between an amine compound and phosgene. As is well-known, phosgene is a reactive compound having a high selectivity, but phosgene has a strong toxicity, and a strict control is necessary when handling this compound. So long as the preparation using phosgene is continued, the risks of damage caused by a leakage of phosgene cannot be avoided.
As the means for preparing an isocyanate compound from an amine compound without using phosgene, there has been proposed a process in which an amine compound is reacted with urea and an alcohol to form a urethane compound and the urethane compound is thermally decomposed in the gas phase (Japanese Unexamined Patent Publication No. 59-205,352 and Japanese Unexamined Patent Publication No. 59-205,353). According to the example of this process, the first step is conducted under a pressure of 6 to 8 bars and the second step is conducted at a reaction temperature of 410.degree. C. in the gas phase, and therefore, the equipment cost of this process is high.
A process can be considered in which dimethyl carbonate is used instead of urea and the alcohol. Compared with phosgene, dimethyl carbonate is a safe compound and a special toxicity-removing equipment or inspection is not necessary. Moreover, dimethyl carbonate is prepared from cheap compounds, that is, methanol and carbon monoxide (see, for example, Japanese Patent Application No. 61-210,568, Japanese Patent Application No. 61-215,178 and Japanese Patent Application No. 61-215,179).
Accordingly, if an isocyanate compound can be obtained by reacting an amine with dimethyl carbonate to form a urethane compound (first step) and thermally decomposing the urethane compound (second step), an economically advantageous process that can take the place of the phosgene process will be provided.
This reaction is expressed by the following reaction formulae: ##STR1##
Several processes have been proposed in connection with the first step.
The speed of the reaction between a dialkyl carbonate and an amine compound is very low at a mild temperature under a mild pressure, and a catalyst is necessary to make the process practically utilizable.
The process usually a Lewis acid as the catalyst is disclosed in Japanese Examined Patent Publication No. 51-33,095. In the examples of this patent publication, aromatic amines and aliphatic amines are disclosed, but, since the amounts of the catalysts used are large and expensive uranium and antimony are used as the catalysts, the process is not advantageous from the economical viewpoint.
There has been proposed a process in which a urethane compound is prepared by reacting a carbonic acid ester with an amine compound in the presence of a base catalyst (Japanese Unexamined Patent Publication No. 54-163,527). Only examples using aniline which is an aromatic amine are disclosed in this patent publication. In this process, N-methylation as the side reaction is preferential to the urethane-forming reaction, and therefore, the yield of the intended urethane is low.
Another process has been proposed in connection with the reaction to be carried out in the presence of a base catalyst (U.S. Pat. No. 4,395,565). Two examples using aniline which is an aromatic amine are disclosed as the example using dimethyl carbonate in this patent publication.
In one of these examples, the reaction is carried out at a temperature (120.degree. C.) higher than the boiling point (9.degree. C.) of dimethyl carbonate under an increased pressure in an autoclave, but the conversion after 5 hours is only 40% and the methylation product is formed in an amount that cannot be neglected.
In another example, 18.5 g (0.199 mole) of aniline, 22 g (0.244 mole) of dimethyl carbonate, 1.24 g of sodium methylate and 25 ml (17.4 g) of methanol, the total amount being 61.4 g (the concentrations of the components other than methanol are 3.2 moles/kg, 4.0 moles/kg and 2.0% by weight, respectively) are reacted at 70.degree. C. under atmospheric pressure for 5 hours. The conversion of aniline is only 15.3% and the space time yield is not satisfactory.
In this patent, it is taught that a temperature range of 100 to 140.degree. C. is especially preferred.
In the conventional processes using a base catalyst, dimethyl carbonate is reacted with an amine only in a low yield or space time yield.
Also in connection with the reaction of the second step, several processes have been proposed. For example, Japanese Unexamined Patent Publication No. 59-205,352 and Japanese Unexamined Patent Publication No. 59-2057353 (U.S. Pat. No. 4,596,679) propose a process in which decomposition is carried out at a reaction temperature higher than 400.degree. C. But the equipment cost is high because the reaction is preformed in the gas phase, and thus the process is economically disadvantageous.
As the process in which the reaction is carried out in the liquid phase, for example, Japanese Examined Patent Publication No. 57-45,736 (U.S. Pat. No. 4,081,472) and Japanese Unexamined Patent Publication No. 51-19,721 (U.S. Pat. No. 3,919,279) propose a process in which an isocyanate compound is obtained by thermally decomposing a urethane compound in a high-boiling-point solvent by using a metal or metal salt catalyst. But the catalyst used is a metal or metal salt free of manganese, molybdenum or tungsten and the amount of the catalyst used is relatively large, and since a method in which the catalyst is charged in the high-boiling-point solvent simultaneously with the urethane compound is adopted as the method for adding the catalyst, the reaction efficiency is still low.
Although Japanese Unexamined Patent Publication No. 51-19,721 (U.S. Pat. No. 3,919, 279) discloses a process in which thermal decomposition is carried out by using the above-mentioned metal or metal salt as the catalyst, a practical working thereof is economically disadvantageous.
Furthermore, these patent publications disclose only examples directed to the preparation of aromatic isocyanates.
Under this background, development of a process for preparing aliphatic isocyanates having a low reactivity, especially alicyclic isocyanate, economically advantageously is desired.